The onboard network supplied from the battery and the charger operates as an IT system: the first earth fault does not interrupt the supply, but it reduces the insulation margin and exposes staff to touch voltage at the second fault. The RN 5897/010 insulation monitor of the VARIMETER IMD family continuously measures the insulation resistance against the vehicle body in DC, AC and mixed networks, signals the drop at a pre-alarm and an alarm threshold, and passes the trigger signal to the fault location system.
The measuring circuit accepts the large leakage capacitance typical of circuits with converters and filters, and setting the maximum capacitance shortens the response time. The datasheet gives a classification to EN 50155 with temperature class OT1 and vibration and shock resistance to EN 61373.
Selection requires checking the battery voltage against the range of the monitored system and the auxiliary voltage range of the module; for circuits with higher voltage on the converter side the RL / RP 5898 coupling device is used.
The battery string itself is supervised by the UH 9830 symmetry monitor: it measures the voltage difference of the two halves of the string against the centre tap and reports a reversed cell, an internal short circuit or sulphation before the battery loses the capacity needed when the mains fails.
